The D90 has a higher end body (ie. top lcd panel; build quality; buttons; fps); but its a bit old as DSLRs go. The new D7000 looks like a very good camera to look forward to. Wait for the new camera if you don't mind paying a bit more and getting the benefits of newer technologies and features.
The 550D generally benefits from a newer sensor; higher ISO; higher mega pixels
If I did not remember wrongly, that feature is only available in 7D.
Canons wireless system requires at least two wireless-capable flash units in order to work. A master flash unit is attached to the cameras hot shoe (either directly or using the Off-Camera Shoe Cord) and the slave flash unit or units are set up to illuminate the scene as desired. At time of writing, only one EOS camera the EOS 7D can use its built-in flash unit to control slave E-TTL units. All previous EOS bodies with built-in flash require an external wireless E-TTL master device.
Sorry but you did not answer my question. What do you intend to shoot?
This may have implications on what camera you get regardless of whether it's old technology or new technology... furthermore, buying a camera system doesn't just mean buying a camera body- you're investing in lenses which you are likely to use for longer than your camera body.
I think comparing a D7000 to a 550D is abit... er...:dunno: 550D is more or less a high end entry level camera... I believe a D7000 is a semi-pro camera (I might be wrong though). You should be comparing the D7000 to the 50D or 60D... but I think D7000 being newer have quite a bit of features that are quite a bit better than both 50D and 60D.
However you would not really know until you pick up a D7000 and a 50D/60D and test it for yourselves. Sometime it is not how much feature there are in the camera... consider also how much of these features are you going to use, or planned to use. No use getting something with a 1000 features and only plan to use one or two... then spend thousands of dollars on this bit of technology.
Plus sometime it is not how new a technology is... it is how it feels in your hand... how you like the user interface. Because I believe image wise, they are quite alike (similar class DSLR).